Is CVE-2026-54340 being exploited?

No public exploit is currently tracked.

How severe is CVE-2026-54340?

CVE-2026-54340 has a CVSS v3 severity of HIGH (7.5).

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CVE-2026-54340

Description

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.